Aluminium-magnesium-(AlMg—)-alloys of the AA 5xxx-type are used in the form of sheets or plates or strips for the construction of welded or joined structures in ship, automotive and aircraft construction. They are in particular characterised by high strength which increases as the magnesium content rises.
For example, from the article entitled Development of twin-belt cast AA5XXX series aluminium alloy materials for automotive sheet applications by Zhao et al., an aluminium strip is known composed of an AA5182-alloy with an Mg content of 4.65 wt. % which is suitable for use in automotive construction.
Aluminium alloy strips of the AA5182-type with an Mg content of at least 4 wt. % are similarly known from the article entitled Semi-Solid Processing of Alloys and Composites by Kang et al. and from the article entitled Comparison of recrystallization textures in cold-rolled DC and CC AA 5182 aluminum alloys by Liu et al., as well as from US 2003/0150587 A1. The article entitled Hot-Tear Susceptibility of Aluminium Wrought Alloys and the Effect of Grain Refining by Lin et al. concerns round bars in an AA5182 alloy.
DE 102 31 437 A1 concerns corrosion-resistant aluminium alloy sheet, wherein through the addition of Zn in an amount of more than 0.4 wt. % sufficient resistance to intercrystalline corrosion is achieved.
Furthermore, published document GB 2 027 621 A discloses a method for manufacturing an aluminium strip.
AlMg-alloys of the AA 5xxx-type with Mg contents of more than 3%, in particular more than 4%, have an increasing tendency towards intercrystalline corrosion, when exposed to high temperatures. At temperatures of 70-200° C. β-Al5Mg3 phases precipitate along the grain boundaries, which are referred to as β-particles and in the presence of a corrosive medium can be selectively dissolved. The result of this is that the AA 5182-type aluminium alloy (Al 4.5% Mg 0.4% Mn) having particularly good strength properties and very good formability cannot be used in heat-stressed areas, where the presence of a corrosive medium such as water in the form of moisture must be contended with. This concerns in particular the components of a motor vehicle which normally undergo cathode dip painting (CDP) and are then dried in a stoving process, as already due to this stoving process, normal aluminium alloy strips can become susceptible to intercrystalline corrosion. Furthermore, for use in the automotive sector, forming during the manufacture of a component and subsequent operational stressing of the component must be taken into consideration.
The susceptibility to intercrystalline corrosion is normally checked in a standard test according to ASTM G67, during which the specimens are exposed to nitric acid and the mass loss based on the dissolution of β-particles is measured. According to ASTM G67 the mass loss of materials which are not resistant to intercrystalline corrosion, is more than 15 mg/cm2.
Such materials and aluminium strips are therefore unsuitable for use in heat-stressed areas.